CL65 ON ILS TO RWY 9 AT IAH WAS REQUIRED TO TAKE EVASIVE ACTION TO AVOID DEP FROM RWY 15R.

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I WAS THE PF. WE WERE ON AN ILS TO RWY 9 AT IAH; VISUAL CONDITIONS; AND CLRED TO LAND. WHEN WE WERE APPROX 3 OR 4 MI FROM THE RWY; HOUSTON TWR TOLD US TO WATCH FOR AN E120 DEPARTING L TO R IN FRONT OF US. MY FO AND I BOTH SPOTTED THE ACFT AS IT BEGAN ITS TKOF ROLL ON RWY 15R. WE BOTH REMARKED OUT LOUD THAT IF IT GOT OFF THE GND QUICKLY; THERE COLD BE A PROB. SURE ENOUGH; THE BRASILIA ROSE RAPIDLY. AT ABOUT 500 FT AGL; I WAS FORCED TO MAKE A MILD EVASIVE MANEUVER; UP AND TO THE L; IN ORDER TO AVOID A COLLISION. (I WAS STILL ABLE TO LAND ON RWY 9.) THE ENCOUNTER GENERATED AN AUDIBLE TCASII TA ALERT. WE LATER ESTIMATED THAT WE'D PASSED ABOUT 300-400 FT FROM THE BRASILIA. WE DON'T KNOW IF THE OTHER CREW EVER SAW US. AS THEY DEPLANED; 3 OF OUR PAX HAD COMMENTS AND QUESTIONS ABOUT THE 'NEAR MISS.' THIS WAS AN UNCOMFORTABLE EXPERIENCE. IF IT IS CURRENTLY HOUSTON TWR'S POLICY TO SIMULTANEOUSLY LAND ARRS ON RWY 9 WHILE LAUNCHING DEPS OFF RWY 15L OR RWY 15R; I THINK THE PROC SHOULD BE REVIEWED IMMEDIATELY. IF CONDITIONS HAD BEEN SLIGHTLY DIFFERENT (MISSED RADIO CALL; LIGHTER BRASILIA; DARKNESS; ETC) IT MIGHT HAVE BEEN MUCH MORE DIFFICULT TO AVOID A COLLISION.