Natural gas produced from coal or biomass is known as synthetic natural gas (SNG), which is expected to replace compressed natural gas (CNG). In this study, we used an 11-l heavy-duty CNG engine in a feasibility study of SNG. SNG, which is composed of 90.95% methane, 6.05% propane, and 3% hydrogen, was produced for the experiment and used as fuel to estimate its effects on combustion and emission characteristics. The torque, fuel flow rate, efficiency, fuel consumption, combustion stability, combustion phase, and emissions characteristics obtained using SNG were compared to those obtained using CNG in an engine speed range of 1,000-2,100 rpm under full load conditions. In addition, an engine fueled with SNG was given an overall evaluation using the World Harmonized Stationary Cycle (WHSC) emission test. The engine's knock characteristic was analyzed at 1,260 rpm under a full load condition. The results showed that there was no difference in power output. The combustion produced using SNG was more stable than that obtained using CNG because of the hydrogen contained in SNG. CO₂ emission was decreased using SNG because of the low fuel consumption and high H/C ratio. However, the nitrogen oxides (NOx) emission of SNG appeared slightly higher than that of CNG. The engine fueled with SNG produced greater knock prevention. Thus, SNG is an attractive fuel for a conventional CNG engine, without the need for any large change in the engine's operating parameters or structure.