Golden Eagle 421
Air Traffic Control Fully Certified
Other / Unknown
Inflight Event / Encounter VFR In IMC
General None Reported / Taken
Weather
While at the Sector; I was receiving numerous icing reports.  Aircraft X departed northbound; requesting 18;000 FT.  Eighteen thousand feet was not usable due to low altimeters; so I asked the pilot if he wanted 16;000 FT or 20;000 FT.  He chose 16;000 FT. The aircraft turned around in the adjacent airspace due to severe icing and was handed back to me at 13;000 FT; my control.  I descended him to 11;300 FT; my lowest MIA; but he could not maintain altitude due to icing. I asked him if he had the terrain in site; and he said no. He was IMC. I informed my supervisor that I had an emergency situation.  I kept the pilot updated on the MIA and when he would be able to get lower.  At one point he was 1;500 FT below my MIA.  I advised him that we have a 2;000 FT buffer in mountainous terrain so he could be as close as 500 FT to an obstacle.  I issued a heading for him to fly to get to lower terrain sooner.  I then asked my supervisor to pull up a terrain chart so I could estimate his proximity to the closest obstacle.  I informed him that he was south of the highest obstacle that we depicted on the chart and kept assigning the lowest altitudes possible.  He was able to get his departure airport in sight about six miles out. He chose to do a visual approach.
An Enroute Controller reported that an IFR aircraft was below MIA and unable to hold altitude in an emergency situation due to icing.