B737 FLT CREW DEPARTING RWY 25R AT LAS RPTS CONFLICT WITH ANOTHER B737 ON GAR FROM RWY 19L/R. FLT CREW LEVELS OFF AT TWR'S INSTRUCTION TO ALLOW GAR TO PASS ABOVE AND BEHIND.

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IN THE CONGA LINE FOR TKOF ON RWY 25R IN LAS VEGAS. ACFT AHEAD OF US WAS CLRED FOR TKOF. WE WERE CLRED INTO 'POS AND HOLD; BE READY FOR IMMEDIATE.' TURNING ONTO RWY WE WERE CLRED FOR IMMEDIATE TKOF. I WAS FLYING AND KEPT ACFT ROLLING AROUND CORNER AND CAME UP NORMALLY ON THE PWR. WE WERE HVY. TKOF WAS NORMAL. AT APPROX 100-200 FT AGL WAS TOLD BY TWR TO; 'SHALLOW YOUR CLB' DUE TO GAR ON RWY 19. I REDUCED PITCH TO APPROX 10 DEGS; CLBING AT APPROX 500 FPM. AT 300-400 FT AGL; WAS TOLD TO 'LEVEL OFF FOR TFC ON GAR RWY 19.' WE LEVELED OFF; SAW THE TFC AT 3 O'CLOCK POS; SAME ALT; APPROX 4000 FT DISTANCE; SLIGHT FRONT TO REAR DRIFT. KEPT THE PWR SET AT TKOF THRUST AND RAISED FLAPS TO 1 DEG AND 'UP;' EFFECTIVELY DOING A LEVEL SPD CHANGE AT 400 FT AGL. WATCHED COMPANY TFC MAKE 'L CLBING' TURN TO PASS ABOVE AND BEHIND. TOLD BY TWR TO CONTINUE CLB AS WE PASSED OVER THE END OF THE LAS VEGAS STRIP. SUPPLEMENTAL INFO FROM ACN 754494: CLRED FOR TKOF RWY 25R. AT 400 FT; TWR TOLD US TO LEVEL OFF AND DID NOT SAY ANYTHING ELSE. THEN THEY SAID STOP CLB AND THEN TOLD US ABOUT THE TFC. THE CAPT LEVELED OFF. ONCE THE OTHER ACFT WAS OUT OF OUR AREA; WE CONTINUED THE CLB. ATC DID NOT SAY ANYTHING ELSE. I TOLD THE CAPT AFTERWARDS THAT THIS EVENT SCARED ME. HAD THE OTHER ACFT GONE AROUND EARLIER; THIS COULD HAVE BEEN PREVENTED. MORE HELP FROM TWR. IT WOULD HAVE BEEN SAFER IF THE OTHER ACFT HAD LANDED. CALLBACK CONVERSATION WITH RPTR ACN 754493 REVEALED THE FOLLOWING INFO: THE ACFT LNDG ON RWY 19L/R WAS ON A DIFFERENT TWR FREQ AND THE REASON FOR THE GAR WAS NOT HEARD NOR EXPLAINED. A COLLISION MOST LIKELY WOULD NOT HAVE OCCURRED EVEN IF THE TWR HAD NOT GIVEN LEVELOFF INSTRUCTIONS.