Laptops don't have many removable parts unlike many desktops. Please review this IFIXIT teardown [guide|9515|MacBook Pro 15" Unibody Mid 2012 Teardown]. If you jump down to Step 12 you can see the layout of the chips on the logic board all of them are soldered down.
The terminology you are using has me a bit baffled here. This model has dual graphics both intergraded (part of the Intel CPU chip) as well as discrete (NVIDIA GeForce GT 650M GPU). Are you having problems with the system over heating? Or shutting down?
To answer your direct question on what you can do here you can connect an external PCI case via a Thunderbolt cable which has within it a PCI graphics card which you then connect the external display to. Here is one persons experience [http://www.xlr8yourmac.com/tips/MBP_ThunderBoltVideoCard.htm|Using an Thunderbolt Video Card with a MacBook Pro].