2001 Buick Lesabre

That small leak may become a big leak, if left unattended. Cooling system is supposed to maintain a specified pressure (usually 16 psi). That leak causes the system to lose pressure. As it leaks, you will have lower coolant level which will cause your heater not running efficiently. To make it worse, knowing that you have a leak, you may have filled up with water instead of coolant, thus reducing the cooling effectiveness. It sounds like you could have a clogged radiator, stuck thermostat or the radiator cooling fan is not working, there is an outside change you have a blown head gasket as well but to be sure lets run down these guides to help us pinpoint and fix the issue.

https://www.2carpros.com/articles/engine-overheating-or-running-hot

and

https://youtu.be/5zpNjYmmiYY

and

https://www.2carpros.com/articles/car-is-leaking-coolant

Let us know what happens and please upload pictures or videos of the problem.