First, ensure your machine is completely shut down (You can press Control + Option + Shift + Power keys to reset the SMC).
Now, boot your machine while holding the Command + S keys. As the system boots, it should display white text on a black screen, and should eventually stop at a prompt.
At this prompt, enter the following command and press Return: nvram fa4ce28d-b62f-4c99-9cc3-6815686e30f9:gpu-power-prefs=%01%00%00%00
Next, enter reboot, and press Return.
Immediately after running the reboot command, hold down Command + R (if you're running OS X 10.11 or later) to boot into Recovery Mode. If you don't have a Recovery Partition, you can boot from a Mac OS installer USB drive.
Once booted in Recovery Mode, open Terminal, and run the following command: csrutil disable
Reboot the system.
Once the system reboots, it should now successfully boot into your install of OS X. Now, you'll need to download the program found here.
Run the program, follow the prompts, and enter your password when prompted.
Once the tool finishes running, your system should now run without the dedicated graphics, and should work completely normally.