I own and drive a 1990 Buick Century. She's got about 135,000 miles on her, and still runs quite good, the car has been well maintained. Recently though, I noticed that I was getting some smoke coming from the radiator after driving for approx. half an hour or so. I have no temp gauge in the dash, so I do not know the temp of the engine. I haven't noticed any leaks or anything in the radiator. Yesterday I started it up and let it run in the drive way for about 20 minutes, and noticed that when I turned the heater off that that was when the smoke come out of the radiator. Any ideas?

Nothing at all????

How often has the coolant been changed? It is possible that you have some corrosion in the radiator which has progressed to pinhole leaks.