There's no need to remove the key. I suspect the problem is your wheel is rusted to the axle. DO NOT try to beat it off. Been there, done that. A couple of things you can try.
First things first, spray some liquid wrench, WD40 or something along those lines into the keyway slot. And on the back of the wheel. If you have compressed air, spray the fluid where I said, then blow it into the wheel. This will help get the fluid inside the wheel and on the axle.
If you have a parts store close by, and your wheels have two small holes in them, buy you a puller and trying pulling it off.
If that doesn't work, you're going to need to heat the wheel (where it meets the axle) til its red hot. Then cool it off quickly with water. Then use the puller or air hammer.
If you don't have those things, then your local small engine shop should be able to get the wheels off the axle and not charge you an arm and a leg. If it's the tires you're needed to replace/repair, they can do that for you also.