Opening the serial port resets the Arduino. Closing the serial port resets the Arduino. Is there some reason for doing that every time you want the temperature and humidity?
SReaderT1 = SerialPort1.ReadLine()
SReaderT1 is a String.
TemperatureHumidity = SReaderT1
TemperatureHumidity is an Integer. You can't assign a String to an Integer.
Even if you could, which float value in the string do you want TemperatureHumidity to contain? The temperature OR the humidity.
This is REALLY a dumb name for a variable. As well as being the wrong type.
If you are just adding the serial data to the list box, why not just add the String?