I might have put too many polygons in the mesh and now it won't compile, so I need to lower the polygon number. Is it possible to do something to create an effect which works the opposite as the subdivision surface, but keeping the smooth surface?
You can use the Decimate modifier: here's the documentation.
It has three different ways of operation: Collapse, Un-Subdivide and Planar. Choose the one the most suits your needs and change the parameters until you reach the right compromise between smoothness of the surface and face count (which is updated live in the modifier box).
Click "Apply" when you are happy with the result. Please note that, if you are using UV maps, you should use the Planar mode and select Delimit: UV.