Posted by sean at 170.148.10.46 on May 22, 2003 at 13:18:38:
Marvin Monroe was killed off in between two seasons, the season that ended with Burns being shot, and the season that began with Burns being shot.
In the episode where Burns is shot, he's taken to the Doctor Marvin Monroe Memorial Hospital, that's how we were told.
I believe Groenig said they did it because they didn't like the character.