Use this app: Button Savior (non-rooted version)
or if your phone is rooted: Button Savior (rooted)
It puts a customizable menu on your screen which you can have always there, or set whatever gesture or button press to bring it up. One of the button options is the power button. It also has options to use the volume buttons to "wake" the phone. Just don't let the battery die unless ur phone turns on automatically when u put the battery back in. Button Savior was working great for me on a previous phone... till I let the battery die once... couldn't turn it back on w/out a power button, lol.
Note: if u are rooted and ever need to get to the bootloader screen or custom recovery (normally done w/ some combination of holding the power button and one of the volume buttons). You can still get to them using command prompt commands on ur computer: cd /(file name your platform-tools folder is in)/platform-tools *press enter*
adb reboot bootloader (but u won't be able to select anything w/out the power button, lol) OR adb reboot recovery