My apologies, I thought I was clear on the subject. The shield is the ethernet shield.
When I connect the ethernet shield on the mega the rfid stop working. It won't read any tags. This is without any code yet for the ethernet shield.
I bent the pin 2 on the ethernet shield so it won't connect to the mega's pin 2 when I stack them together. And then I connect the RFID straight to the mega's pin 2. I was able to read tag but with some problems- rfid would beep for no reason; the reading range of the rfid is gone; and it takes a couple of taps before the rfid recognizes that a tag is present.
I had to use pins 2 & 3 because the rfid uses (data/clock) interrupt to read tags. My project is a simple access control. When the user scan his/her tag, it then checks in the database server to check if the tag is valid and unlock the door.
I hope this is enough details. Thanks again!