hie team .. please assist. is a 12V, 2A Power supply safe to use for my SIM900 gsm/gprs shield
God only knows what your particular SIM900 shield is but, typically, they run on 5v, so it would appear that you have 7v too many and it might be a good idea to think about voltage regulators.. The 2A isn't a bad start, though.
this is the exact module am using
The shield is more or less identical to my Geetech, from whence you may get some better information. It is something of a power hog. The barrel jack is for 5v. The switch beside it enables you to get power from Arduino, which is a really bad idea if you are relying on Arduino's regulators. If you put a decent 5v regulator on the 24w power supply you have, you should be able to power both shields nicely.
I understand this is a 2G device, and you might need to check this is OK for you.