Ridge Vent has Condensation on the underside after a heavy rain
Condensation is indicative of poor airflow. A ridge vent is great, but if there isn't adequate venting at the eaves, they're all but useless. A good roofer would have checked for sufficient venting at the eaves. But, it's possible that you're fine there and that the problem is, as Joe mentioned, that the vents are blocked by insulation. Soffit baffles will solve that.
You mentioned a hard rain, which, interestingly, shouldn't have anything to do with condensation. Are you sure it's condensation and not water penetration? Did you have a hot day yesterday and a very cool night/morning?