2001 Dodge Neon car overheating but not sure why

This car started overheating when I covered up a crack it had on the radiator, so I took the thermostat of, but it still would overheat, the catalytic converter heats up real bad, not sure if that's normal. I can tell when its overheating because when I try to give it gas its a little rough and I have to push a little harder on it. I think that the coolant isn't been circulating correctly, it might clogged up, took it to a mechanic he said everything looks normal, yes the fans turn on. Please help