A VFR BEECH 99 AIRLINER DEPARTING HENDERSON HAS AN NMAC WITH AN INBOUND VFR BONANZA 1 MI SW OF L15 WHILE UNDER CTL OF RADAR DEP CTLR AT LAS; NV.

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WE WERE DEPARTING L15 (HENDERSON EXECUTIVE ARPT) AS THE 8TH OF 9 BEC99'S TO DEPART IN SEQUENCE TO PROCEED ON A CHARTED AND APPROVED DEP COURSE ('SUB 1'). WE WERE CLRED TO DEPART ON THE SUB 1 BY LAS DEP (OUR STANDARD DEP FOR A GRAND CANYON TOUR) WHICH INVOLVES REMAINING LEVEL AT 3500 FT MSL UNTIL CLRED FOR HIGHER BY LAS DEP. AFTER TKOF; WE CLBED STRAIGHT OUT ON A HDG OF 180 DEGS AND AT 2900 FT MSL TURNED R TO OUR APPROVED HDG OF 360 DEGS (THIS KEEPS US CLR OF THE LAS CLASS B AIRSPACE). IN THE TURN; LAS DEP INSTRUCTED US TO 'TURN IMMEDIATELY TO 270 DEGS...INBOUND TFC; A BEECH BONANZA AT 11:30-12 O'CLOCK POS; 3500 FT.' I WAS THE PF AND DID AS INSTRUCTED. BOTH THE CAPT AND I LOOKED FOR THE TFC AND COULD NOT SEE HIM. AT THIS POINT; WE WERE LEVEL AT 3500 FT MSL AND WELL INTO THE CLASS B AIRSPACE (DUE TO FOLLOWING ATC INSTRUCTIONS TO TURN TO 270 DEGS). THE BE-C99 IS EQUIPPED WITH TCASI. AFTER APPROX 5 SECONDS; WE RECEIVED A TA. WE STILL DID NOT SEE THE ACFT AND CONTINUED OUR TURN TO 270 DEGS. THERE WERE NUMEROUS TARGETS ON TCASI. FINALLY; THE BONANZA APPEARED LOWER AND DIRECTLY IN FRONT OF US. AS PF; I TOOK EVASIVE ACTION AND TURNED HARD TO THE L TO AVOID HITTING THE BONANZA. IF WE HAD CONTINUED ON OUR NORMAL DEP PROC AFTER TKOF; THE BONANZA WOULD HAVE PASSED WELL BEHIND US AND WOULD HAVE BEEN NO FACTOR. HOWEVER; THE INSTRUCTIONS TO TURN TO 270 DEGS TURNED US RIGHT INTO HIS FLT PATH AND ALMOST CAUSED A COLLISION. QUICK THINKING AND FAMILIARITY WITH THE CONGESTED AIRSPACE ALLOWED ME TO RESPOND APPROPRIATELY. DEP SHOULD THINK TWICE BEFORE GIVING A HDG TO AVOID TFC.