A method of using a feedstock gas reactor is described. A hydrocarbon, such as methane, is chemical decomposed in the feedstock gas reactor using heat of combustion generated from the combustion of a combustible gas. A mixed product stream is extracted from the feedstock gas reactor. The mixed product stream comprises hydrogen, carbon, and water. At least a portion of the one or more combustion product gases are vented from the combustion chamber. At least some of the carbon is activated using the vented one or more combustion product gases. At least some of the activated carbon is recycled to the feedstock gas reactor.