A dual vessel reactor and a method of carrying out a reaction using a dual vessel reactor are provided using a non-condensable gas to substantially isolate the inner vessel from the outer vessel during the reaction and limit the heating of the outer vessel when steam from the inner vessel condenses on the interior surface of the outer vessel. By limiting the heating of the outer vessel through the condensation of the steam or other vapour from the inner vessel, the operating temperature of the outer vessel is kept below an upper threshold of the operating temperature of a seal used to seal the door in the outer vessel.