In order for the Rebels to win the Drawn In mission, the following must occur: "When all 4 power stations have been destroyed and and all heroes are on or adjacent to the entrance, the heroes depart." (In addition, heroes that have been defeated twice don't withdraw, but instead become limited to taking one 3-movement action per activation)
Our game essentially ended as follows: http://imgur.com/cR3ypKk
Gideon is on the entrance, and Dialla and Mak are clearly adjacent to him. Mak and Dialla were both wounded and had finished their turn, Gaarkhan was twice-defeated and could only use his single movement, and Gideon was close to wounding, which would end the game. The Imperials had the ability to hit him with almost everything on the board and there was basically no way he could survive.
As the Imperial, I interpreted the victory condition to mean that all four heroes had to be adjacent to the exit at the same time in order to win, and consciously plugged up the location as best I could. Was this the correct interpretation or could the Rebels win by either: A. Getting as close as possible to adjacent as enemy figures would allow; B. Move Gaarkhan into an occupied adjacent square, even though he would not normally be able to remain there; or C. Remove the Rebels individually as they arrive at the entrance?
I'm fairly certain my interpretation of an Imperial victory is correct, but I could find no definitive answer.
Also, that was a hell of a mission, and went right down to the wire as can be seen.