You’ll need to watch here. The older 36’s have a ridiculous 2 piece axle where the allen key head is screwed into the axle body itself, on the opposite thread to the main axle. This means if you try to remove the allen bolt head, the axle end cap unscrews, leaving the axle still in place 🙁
The best way to remedy this is as above, but screw the axle cap in with some loctite on it, leave to cure and then remove both. Of course the good bit about this set up is that if you round the Allen head you can easily replace it.
However, it does sound like you have a different problem where the allen bolt head has rounded and the axle is still stuck fast. If thats the case you could try the methods above with boiling water etc.