FLC OF A SAAB SF340 DSNDED AND TURNED OFF OF ASSIGNED HDG DUE TO ACFT ICING AND INABILITY TO HOLD ALT CAUSING LTSS WITH OPPOSITE DIRECTION TFC AT A LOWER ALT.

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WE WERE FILED AT FL200. #2 ENG WAS ON DEP PWR. CLB WAS SLOW AT 140 KTS. TRACE OF ICE BEGAN ABOUT 15000 FT. AT CRUISE; LEVELOFF AIRSPD WAS ABOUT 155 KTS. WITHIN 1 MIN IT WAS DOWN TO 145 KTS WITH ALL ICING EQUIP ON. I ASKED THE FO TO CALL ATC AND TELL THEM WE NEEDED A DSCNT TO FL180. ATC RESPONDED WITH AN 'UNABLE DUE TO OPPOSITE TFC AT 9 MI; FL190.' WITHIN A FEW SECONDS; AIRSPD DECREASED TO 140 KTS AND I AGAIN ASKED FO TO CALL ATC WITH THE NEED TO DSND AND WE COULD TAKE A TURN; IF NECESSARY. ATC AGAIN SAID UNABLE DUE TO OPPOSITE TFC. WITHIN A FEW SECONDS; AIRSPD WAS 135 KTS AND DECREASING. WANTING TO AVOID A STALL AND UNCTLABLE ACFT DUE TO ICE; I CALLED ATC; TOLD THEM WE WERE UNABLE TO MAINTAIN OUR ALT DUE TO ICING AND WERE LOSING AIRSPD AND THAT WE WERE DSNDING TO FL180 NOW. I TURNED 20 DEGS L AND DSNDED TO FL191 RAPIDLY; AT WHICH TIME THE TCASII WENT OFF WITH A CLB ALERT -- WANTING US TO CLB AT 4000 FPM. I HAD HEARD ATC TELL THE OTHER TFC TO CLB TO FL210 IMMEDIATELY. I HELD THE ALT THAT I HAD (DURING THE TCASII ALERT) FL190 PLUS 500 FT; UNTIL THE OTHER ACFT HAD PASSED THEN DSNDED TO FL180. (I SHOULD HAVE DECLARED AN EMER; BUT ALL THIS HAPPENED FAIRLY RAPIDLY.) FL180 WAS STILL TOO HVY ON ICE SO WE WENT TO 16000 FT; AND ON TO RDU (FROM JFK). A MALFUNCTION OCCURRED AT FL180 WITH THE BOOT SYS AND THAT MAY HAVE BEEN A CONTRIBUTOR TO OUR INABILITY TO SHED MORE ICE.