Plug the heat gun into a socket as close to the doorway as possible. Point the heat gun at one area of the ice and 8 to 12 inches away.
Turn the heat gun on and leave it pointing at the ice for 2 or 3 minutes. Turn off the heat gun and set it aside.
Place the chisel against the area of ice that you just melted a little with the heat gun. Hit the handle of the chisel with the hammer to break up the ice.
Toss the ice chunks away from the door. Do not leave them near the door.
Repeat until all the ice is removed from the doorway.