DURING VMC SUMMA DEP CLBING THROUGH 12000 FT RPTR EXPERIENCED TCASII AND NMAC WITH CESSNA 180.

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FLT DEPARTED N ON THE SUMMA 5. AT 8 DME; WHILE IN THE TURN TO 070 DEGS WE WERE CLRED TO CONTINUE TO HDG 160 DEGS TO INTERCEPT THE SEA 143 DEG OUTBOUND. ON THE 160 DEG HDG; 4.5 DME E-SE OF SEA VOR AT APPROX 12000 FT AND CLBING AT APPROX 2000 FPM; A YELLOW TFC ALERT APPEARED ON OUR TCASII. IT SHOWED +05 AND WITHIN THE INNER CIRCLE OF THE 15 DME SELECTION. MY LINE OF SIGHT RAISED UP TO LOOK OUT IN THAT SPECIFIC AREA AND I SAW THIS SINGLE ENG ACFT AT OUR 11:30 POS AND SLIGHTLY HIGHER. I SIMULTANEOUSLY INCREASED OUR RATE OF ASCENT AND LOWERED THE L WING INSTINCTIVELY TO MISS HIM. HE DOVE TO HIS R AND PASSED WITHIN 100 FT OF OUR L WING. WITH OUR RATE OF CLB APPROX 3000-3500 FPM THE TCASII LOUDLY PROCLAIMED 'TFC; TFC.' THE TCASII WAS A COMPLETE RED ARC WITH THE EXCEPTION OF THE 4000 FPM UP AND ABOVE PORTION. AS HE WENT BY WE COULD SEE THAT IT WAS A RED AND WHITE CESSNA 180. WITHOUT OUR ALARM WE 'PROBABLY' COULD HAVE READ HIS NUMBERS. AS PAX DEPLANED IN RNO; 2 ALARMED PEOPLE LEFT THEIR PHONE NUMBER AS THEY HAD SEEN THE CESSNA 180 PASS BY.