Gotcha! Yeah, your best bet is to throw it in an ice bath. A room temp commercial keg has a ton of thermal mass, and it's going to take a while to cool down.
BTW, commercial kegs are delivered cool with the CO2 dissolved into the solution. When your keg was warmed to room temp, much of that CO2 came out of solution and is now in the headspace of the keg.
You can quick cool this thing, but it is going to take a good 48 hours under 40F to get all of that CO2 dissolved back into the beer. If you quick cool this and serve it tomorrow, get ready for REALLY foamy beer from the CO2 in the headspace of the keg agitating the nucleation sites on the surface of the beer. You are going to get pours that are 50-75% foam with actual beer that tastes almost flat, no matter how slowly you pour it. Nothing you can really do but give it time.
Primary #1 - Midnight Ryeder (Midnight Wheat and Rye)
Primary #2 - Florida Weiss
Primary #3 - Kane-DOH APA (Honey Citra APA)
Secondary #1 - Downtown Flanders Brown (brewed August 2012)
Keg #1 - Raspberry Florida Weiss
Keg #2 - Cinnamon Raisin Cider
Keg #3 - NONE!
Bottled - NONE!