H2S, a toxic gas present in raw natural gas and in oil fields, is converted to sulfur using the Claus process in sulfur recovery units. The low and fluctuating demand and sale price of sulfur and its increasing production due to the discovery of new sour fields have mandated optimization studies to reduce the production cost. Natural gas (mainly methane) is continuously cofired with acid gas (H2S and CO2) in the Claus furnace to maintain temperatures above 1050 degrees C, which adds to the operating cost. This study aims at reducing the usage of natural gas in sulfur recovery units using a detailed reaction mechanism for Claus feed combustion, while maintaining the desired furnace temperature. An optimal combination of feed-preheating using natural gas and its cofiring in the furnace is determined through furnace and fired heater simulations. A model for the feed preheater is developed using plant measurements. Through simulation studies by varying natural gas flow rates into the Claus furnace and to the air preheater, it is observed that (a) a considerable amount of natural gas can be saved to maintain the required furnace temperature, if air is preheated to a higher temperature rather than natural gas cofiring (also validated in an operational sulfur recovery plant); (b) a reduction in natural gas to the furnace improves the sulfur recovery efficiency; and (c) the destruction of aromatic contaminants (that are harmful for the catalysts present downstream) in the furnace is enhanced with decreasing natural gas cofiring and increasing feed temperature. The rate-of-production analysis is also presented to understand the chemical effects of natural gas in the furnace.