B737-5 FLT CREW ENCOUNTERS A VEHICLE ENTERING THE RWY DURING LNDG ROLL AT DEN.

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ON SHORT FINAL; AFTER BEING CLRED TO LAND; THE FO AND I OBSERVED A SNOWPLOW SLOWLY MOVING TOWARDS OUR LNDG RWY FROM AN UNNAMED SVC ROAD FROM L TO R. ALTHOUGH NOT VERBALIZED; WE BOTH WONDERED IF IT WAS GOING TO HOLD SHORT; AND I MENTALLY PREPARED TO GO AROUND. THE PLOW PROCEEDED TO STOP JUST SHORT OF THE RWY; SO WE BOTH CONCLUDED THAT HE HAD SEEN US AND WOULD CONTINUE TO HOLD SHORT. ON ROLLOUT AT SOMEWHERE BTWN 60-100 KTS I OBSERVED THE PLOW SLOWLY ENTERING THE RWY ENVIRONMENT. I IMMEDIATELY TOOK THE CTLS FROM THE FO AND APPLIED SIGNIFICANT REVERSE THRUST AND MANUAL BRAKING TO AVOID CONTACT WITH THE PLOW. AT ABOUT THAT TIME IT LOOKED AS IF THE DRIVER HAD SEEN US AND WAS ACCELERATING TO CLR THE RWY. I WOULD GUESSTIMATE WE STOPPED THE ACFT SOMEWHERE WITHIN 100-300 FT OF THE SPOT WHERE THE PLOW WAS QUICKLY EXITING THE RWY. ONCE STOPPED; WE RPTED THE INCIDENT TO TWR BEFORE EXITING THE RWY AT THE 1ST HIGH-SPEED. WE THEN TAXIED TO THE GATE WITHOUT INCIDENT. ABOUT HALFWAY TO THE GATE THE TWR SUPVR CAME ON THE FREQ AND ASKED WHAT HAD HAPPENED AND IF WE HAD ALMOST HIT AN ANIMAL ON THE RWY.