Abstract: A method of producing a hot, carburized metallized iron product in a generally vertical shaft furnace having an upper reducing zone in which iron oxide reacts with a gaseous reductant, and a lower carbon control and product discharge zone, including: establishing a gravitationally descending iron oxide burden in the furnace; reacting hot gaseous reductant with the descending burden to form a metallized iron product and a reacted top gas; and injecting a hydrocarbon gas mixture consisting of hot reformed endothermic gas and cool natural gas to the product discharge section of the shaft furnace; whereby the carbon content of the metallized iron pellet product is controlled by mixing the endothermic gas and natural gas in the proper ratio to balance the endothermic and exothermic reactions within the discharge zone of the furnace.