I am trying to have a seperate power supply for the mega and keep hardware disconnected from pc. Is this possible?
sure.. actually you dont need a pc connection to run any of the arduino boards.. as written in documentation your board supports
Input Voltage (recommended) 7-12V
Input Voltage (limits) 6-20V
so you can use an adapter in this range..
for your motor shield it is documented as
Operating Voltage 5V to 12V
Motor controller L298P, Drives 2 DC motors or 1 stepper motor
Max current 2A per channel or 4A max (with external power supply)
so if you use an adapter that can support the current needed by your motors you can use both arduino and motor shield with the same power source