How do I eliminate a harsh lighting issue on my cylindrical glossy model? I am new to blender and have recently created my first model.  I created the mesh and then textured it, added a world environment texture, and rendered it only to find a very dark harsh "shadow" around the rear of my model.  However, the shadow only appears when I add a face to the back of the model to complete the whole mesh.  I have included pictures so you can see what I am talking about.  

Without the rear face


With the rear face


In the first picture you can see the model's cylindrical body WITHOUT it's rear face resulting in no shadow.  However, in the second picture, a face was added to the rear of the model resulting in a harsh shadow as shown.
I am using blender version 2.73
It appears that you are using a Subsurf modifier, and a flat many-edged face on the back. That would be called an N-Gon, and is highly frowned upon in Blender as it brings a host of issues into the picture. Quads (faces with 4 edges) are much prefered, as it they work best for almost all of the relevant calculations done by Blender.

This particular issue can be resolved by adding an edge-loop near the face on the back by pressing Ctrl+R