NEAR LOSS OF ACFT CTL SUFFERED BY SMT PLT DURING INFLT ENCOUNTER WITH ICING CONDITIONS.

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DURING WX BRIEFING I WAS TOLD LIGHT TO MODERATE RIME ICING WAS EXPECTED OVER THE SIERRAS BELOW 18000 FT. WHILE MY ACFT IS 'KNOWN ICE' EQUIPPED AND MAINTAINED I AVOID ALL FLTS INTO ICE. I FILED FOR AND EXPECTED TO BE ABOVE THE ICING CONDITIONS WELL BEFORE REACHING THE SIERRAS. IN FACT; CLB CLRNCS WERE SUBSTANTIALLY SLOWER THAN USUAL AND I HIT SEVERE ICING CONDITIONS BEFORE I REACHED 18000 FT. THE ACFT BECAME VERY DIFFICULT TO CTL AND I RECEIVED SEVERAL ADMONITIONS FROM THE CTLR THAT I WAS NOT MAINTAINING MY DIRECTION OF FLT. I EXPLAINED TO HIM THAT I HAD GREAT DIFFICULTY CTLING THE PLANE DUE TO SEVERE ICING -- PROBABLY MOSTLY ON MY PROPS. (THEY ARE IN FACT HEATED). USING FULL ENG PWR I WAS SLOWLY ABLE TO GET ABOVE THE CLOUDS (AT ABOUT 18200 FT) AND CONTINUED MY FLT UNTIL I WAS ABLE TO DSND WITHOUT GOING THROUGH CLOUDS AND LAND IN LOVELOCK; NV; ('LCL' -- DERBY FIELD) LATER I WAS ABLE TO COMPLETE MY FLT TO SLC; UT. THERE WERE 2 IMPORTANT LESSONS HERE FOR ME. FIRST -- ONE CAN'T EXPECT TO BE CLRED TO ALTS FILED AS RAPIDLY AS EXPECTED AND SECOND; ICING CAN BE A LOT MORE SEVERE THAN FORECAST.