arduino mega overheat when connected to TFT 7" and TFT shield
7" LCDs need a lot of current. They need it for the back light.

That current comes from the 5V pin.  That 5V pin is supplied by a linear regulator that has limited current capacity.  As you draw more current it gets hotter and hotter and eventually shuts down.

You will need to power your LCD screen with another external 5V source. The best solution may be a powerful (2A) phone charger (USB) which you then plug direct into the USB port of the board thus bypassing the weak regulator.