Is it normal for Nikon D5100 to feel slightly warm at the grip area?

Let say after 10~15min usage

Well for one, your hand is there for 15 minutes. It's bound to heat up

secondly, the battery is located in the grip which could have something to do with it. If it gets really hot, then you have a short somewhere in the battery

Do you understand what's actually happening inside a digital camera? Probably not.

Look, billions of electrons are flowing around in there so you can have pictures. Those electrons moving around create heat - that's what you're feeling. Yes it's normal.

If the camera is working hard (eg shooting video) then the battery will get a little warm. Nothing to worry about.