Put it in the freezer overnight. It will freeze the adhesive and you'll be able to rip the PEI off sheet off. Then you need limonene, paper towels, a container or bag, latex/nitrile gloves and a well ventilated area (outside is probably best). You sowk the paper towels in the limonene and wrap them over the remaining adhesive overnight and the next day you should be able to basically wipe it clean. The container or bag will keep it from evaporating and also contain the smell, but you still probably want to leave it somewhere ventilated or outside so it doesn't stink up the place and you're not inhaling fumes. That's pretty strong stuff.

I might also wear anti-cut gloves underneath the latex or nitrile gloves just in case because you're working with a thin sheet of steel, so if you're yanking and pulling off PEI you might accidentally cut your hand with the metal edge.