No it "vents" heat through the oven vent. That is normal (unless it is coming from other than the designated vent area).
** Is there a valve I can close? **
No there isn't and if you blocked the oven vent you would no longer be able to bake properly. A vent is required for proper air circulation in the oven which is needed for even baking and temperature control.
** Oven never gets to temp **
A gas oven should be able to get near set temperature even if the door was wide open. The minimal amount of heat allowed to escape through the oven vent is not the cause of your problem.
Someone will need to look into why the oven isn't heating properly or heating and then stopping for whatever reason.