I hope you can see from some of the other responses, that in most cases condensation IS eliminated. Assuming that there is a good perimeter seal with the magnets pulling together with no gaps, the cause may be due to a certain amount of residual dampness in the window frame, or water may be coming in from OUTSIDE. These are just possibilities. We have had months of very wet weather! If the panels are removed during the summer months, the window frame will have a chance to dry out thoroughly, and you shouldn't have this problem next winter.
Regards
Richard Cutts