You probably want to use Smoothing Groups for this: https://developer.valvesoftware.com/wiki/Smoothing_groups
Select the Face Edit Tool, which brings up the Face Edit Sheet
Then, select all the faces you want to look smooth
Click on Smoothing Groups in the Face Edit Sheet
That will bring up a window with tabs numbered from 1 to 24, assign the selected faces to one of these tabs. (It doesn't matter which one)
When you compile your map, It should look smooth as a baby's butt.
Hope it helps.