THE PILOT STATED THAT JUST AFTER TOUCHDOWN THE WHEELS WENT INTO A SNOWDRIFT ABOUT 2 FT DEEP WHICH PULLED THE AIRPLANE SIDEWAYS. HE APPLIED FULL POWER TO MAKE A GO-AROUND, THEN HIT ANOTHER SNOWDRIFT ABOUT 12-16 IN DEEP. THE AIRPLANE THEN SLID OFF THE RUNWAY. THE OPERATOR'S DISPATCHER STATED HE TELEPHONED THE AIRPORT & WAS TOLD 'TO THE BEST OF HIS KNOWLEDGE THE RUNWAY CONDITION WAS GOOD'. THE AIRPORT PERSON RESPONSIBLE FOR PLOWING THE RUNWAY STATED THAT THE SNOWPLOW HAD BROKEN DOWN, AND TOLD THE DISPATCHER TO TELL THE PILOT TO TAKE A GOOD LOOK AT THE RUNWAY BEFORE LANDING. IT WAS TOO DARK TO CHECK THE RUNWAY EARLIER IN THE MORNING. THERE WAS NO NOTAM IN EFFECT PERTAINING TO THE RUNWAY CONDITION. THE ALASKA SUPPLEMENT FLIP CAUTIONS ' RUNWAY CONDITION NOT MONITORED, RECOMMEND VISUAL INSPECTION PRIOR TO USING.' THE ACCIDENT OCCURRED 15 MINUTES AFTER THE BEGINNING OF CIVIL TWILIGHT, AND 44 MINUTES BEFORE OFFICIAL SUNRISE. THE PILOT STATED BECAUSE OF THE LIGHT CONDITIONS HE DID NOT SEE THE SNOWDRIFTS BEFORE BEFORE HITTING THEM.