This study presents the effect of natural gas (NG), composition on carbon monoxide (CO) yield in the products of an autothermal reforming (ATR) process. Since NG is a mixture without definite composition, a computer routine is coded, which is able to randomly generate various compositions whose individual constituents fall within the defined ranges Then each NG is analyzed based on the AIR process within optimal ranges of the air-to-fuel ratio (AFR) and water-to-fuel ratio (WFR), which were obtained from a typical composition of NG under the constraints of maximum hydrogen production, minimum carbon monoxide yield, and free from solid carbon in the reformate. Finally, the correlation between NG composition and carbon monoxide yield is established. This provides valuable information for the development of methods for the clean-up of carbon monoxide.