B777 pilot reported malfunctioning air conditioning system resulted in high cabin air temperatures and a gate return.

Narrative

I reported at XA:25 local time for a XB:25 departure from ZZZ to ZZZZ. The flight attendants had been telling us that it was too hot in the back and we had notified maintenance. We had an MEL associated with the air conditioning system already; but maintenance advised that the plane would cool better after the engines were started and even more so at altitude. After we left the gate on time; we taxied out and received several calls from the FAs advising that it was too hot in the back still. We stopped the plane and the CA called maintenance again. We were able to reset the air conditioning system and it seemed to briefly help. Within a few minutes; the temperature started to climb again and we made the decision to return to the gate. It took several minutes and many calls to have the gate ready for our return. While waiting for the gate; we were advised of two passengers that seemed to be having problems with the heat in the back; one may have lost consciousness. Eventually we made it to the gate and deplaned.I believe that we as the flight crew underestimated the severity of the heat in the back of the plane. An additional maintenance item popped up and made the situation worse. We could have probably dealt with the heat earlier and deplaned before we did.