If the metal plenum is sweating, then there is an insulation problem. They might have used just a sheet metal plenum with out any internal insulation...in which case they should have wrapped it on the outside. I would take some pictures and give the installing company a call.

Hope this helps

Thanks for your reply, but they did cover the outside of the plenum with insulation. Therefore, the water/condensation is collecting on the outside of the insulation. What can I do about that, and how would I know if there was moisture underneath the insulation as well?

It seems likely to me the problem may be excessive moisture in your crawlspace. If the crawlspace is vented as most are, consider that you have hot humid air surrounding the outside of the building. Now you bring that humid air into the crawlspace where it is nice and cool..The humid outside air reaches saturation point in the crawlspace and anything just slighly cooler like you ductwork is going to sweat. Check the humidity in your crawlspace. Solution could be sealing the crawlspace to installing a dehumidifier. I believe that the the latest trend in new crawlspace construction is to seal and insulate them to prevent moisture and mold problems.

Consider the possibility that outside air is leaking past poorly-installed insulation and water is condensing on the duct. That wets the insulation rendering it useless so you get condensation on the outside vapor barrier.