** Dryer vent ducting is sweating ... This started happening in the summer, when the weather outside was warm. **
Sounds to me like the vent line from the dryer to the outdoors may be partially (or fully) plugged. If the material inside the ceiling is a corrugated design, it probably should be changed to a rigid type material to help prevent lint build-ups.
You can read about dryer venting at the following link: