Need some help for getting my temp graph on google docs

Hello… i’m a beginer here and i don’t really know what to do in order to make my program to run. Any help or advice would be well taken. Here is the code, i got an error for += operand when i try to add my measurements results to my data string:

#include <Ethernet.h>
#include <SPI.h>
#include <Wire.h>


char formkey[] = "dG9HWmNXWjNRdWhBWG5ITlpNeUVBU2c6MQ"; //My Key from Google Docs
byte mac[] = { 0x90,0xA2,0xDA,0x00,0x55,0x8D};  
byte ip[] = { 192,168,1,117};  //The Arduino device IP address
byte subnet[] = { 255,255,255,0};
byte gateway[] = { 192,168,0,1};
byte server[] = { 173,194,70,139 }; // Google IP
int tmp102Address = 0x48;

EthernetClient client;
void setup()
{

Ethernet.begin(mac, ip);
Serial.begin(9600);
SPI.begin();
Wire.begin();
delay(1000);
// start the Ethernet connection:
if (Ethernet.begin(mac) == 0) {
Serial.println("Failed to configure Ethernet using DHCP");
while(true);
}
delay(1000);
Serial.println("connecting...");

}



void loop(){
  
  Ethernet.begin(mac, ip , gateway , subnet);
  delay(1000);
  Serial.println("connecting...");
  EthernetClient client(80);


  String data;
  data+='';
  data+='entry.0.single=';
  data=data+getTemperature();     //ERROR
  data+='&submit=Submit';

Serial.println(celsius,2);

if (client.connect(server,80)) {  
    Serial.println("connected"); 

    client.print("POST /formResponse?formkey=");
    client.print(formkey);
    client.println("&ifq HTTP/1.1");
    client.println("Host: spreadsheets.google.com");
    client.println("Content-Type: application/x-www-form-urlencoded");
    client.println("Connection: close");
    client.print("Content-Length: ");
    client.println(data.length());
    client.println();
    client.print(data);
    client.println();

    Serial.print("POST /formResponse?formkey=");
    Serial.print(formkey);
    Serial.println("&ifq HTTP/1.1");
    Serial.println("Host: spreadsheets.google.com");
    Serial.println("Content-Type: application/x-www-form-urlencoded");
    Serial.println("Connection: close");
    Serial.print("Content-Length: ");
    Serial.println(data.length());
    Serial.println();
    Serial.print(data);
    Serial.println();

  }

  while (client.available()) {
    char c = client.read();
    Serial.print(c);
  }

  delay(15000);

  if (!client.connected()) {
    Serial.println();
    Serial.println("disconnecting.");
    client.stop();
  }

  float getTemperature(){
   Wire.requestFrom(tmp102Address,2);
byte MSB = Wire.read();
byte LSB = Wire.read();
int TemperatureSum = ((MSB << 8 ) | LSB) >> 4; 
float celsius = TemperatureSum*0.0625;
return celsius;
  }
  delay(5000);

}

When you put string literals such as 'entry.0.single=' and '&submit=Submit' in your code, you need to enclose them in double quotes "like this", not single quotes 'like this'.

When posting code please enclose it in [ code ] [ /code ] tags to stop the forum software from munging it. You can add them to your original post by editing it, selecting the code and clicking the # button in the edit window to add code tags. Also please include the complete text of any error message you want help with.

OK, ty for the advice, i tagged the code.
For a string i tried with "text" and he didn't warn me about any error. Right now i`m not at home but i will upload the complete error message code.
Thanks