RS232 Shield question/problem

I have the following system:

Arduino Mega board connected via USB to the laptop. On the laptop I have a program (Modbus simulator) that communicates with the board via USB using Serial Port. Also to the board I have connected Zigbee chip (resides on the breadboard) that uses Serial1 Port on the Arduino Mega port and communicates via RF with another Zigbee chip. The system works fine.

However, when I disconnect the USB cable and connect RS232 shield on the top of the Arduino Mega board the system stops working. I of course try now to communicate using RS232 instead of the USB connection to my laptop.

I believe I don't understand what exactly happens when the RS232 shield is installed on top of the Arduino Mega board. Does it still use the same Serial Port that USB was using when I had it connected to my laptop or there is some different mechanism.

(I have made pictures of these two systems but I was unable to attach them to this post, however. I will be glad to e-mail them to anyone that would like to have a better understanding of what I wrote above.)

Could you try again uploading a photo of the shield ?
Click on "Additional Options...".
And could you give a link to what kind of RS232 shield you bought ? (just copy the url in the post).

Is it this one ?
If so, you are in so much trouble. They use the same pin 0 (RX) and pin 1 (TX) as the Mega itself. So the shield should override the existing path to the usb virtual serial port.
They didn't connect the DTR to the reset, so when you want to upload a sketch, you have to press the reset button.

Why do you have to use a shield ?
Must it be a true RS232 port ?
Could you use a usb-to-ttl-serial port ?