We have air vents in all our rooms. We haven't had the heating on yet (it is 20 degrees C in here). We are in a flat of 4; the other units don't have this problem. The window sills run with water if you touch the windows. We tried leaving top window open a bit in each room, had our check for winter done, but still no luck.
Showers without venting, a clothes dryer that is not connected to a vent or cooking can all cause the windows to sweat. When my wife is canning even our double pane windows start sweating. A dehumidifier can really help reduce the moisture in the home and Eliminate the window condensation or sweating.