The steady-state design and economic evaluation of a polygeneration (POLYGEN) process to coproduce synthetic natural gas (SNG) and ammonia are studied in this work. POLYGEN has been a widely studied topic recently, in which several products could be produced parallel at the same time. One of the two products in this study, SNG, has a composition and heat value very similar to those of typical natural gas, and can be used as a replacement in industrial and home usages. Another product, ammonia, is one of the most important inorganic chemicals in the world, and could be used as the precursor of various kinds of chemicals, as fertilizers, or as a cleaning agent. In the POLYGEN process, the relative production rates for different chemicals could be adjusted on the basis of different market demands, daily usages, and also changing political strategies. In our previous study (Yu, B. Y.; Chien, I. L. Design and Economical Evaluation of a Coal-to-Synthetic Natural Gas Process. Ind. Eng. Chem. Res. 2015, 54, 2339-2352), we illustrated that the SNG production price is lower than the liquefied natural gas importation price in Taiwan. The SNG production price is 10.837 USD/GJ (USD = U.S. dollars) in an SNG-only plant. With the POLYGEN process to coproduce SNG and ammonia, the SNG production cost could become even lower. If 20% of the syngas is used to produce ammonia, the SNG production price will drop to 9.365 USD/GJ, and if 40% is used for ammonia production, the SNG production price will drop further to 7.063 USD/GJ. Thus, although the POLYGEN process leads to an increasing total capital investment, it has positive influences from economic aspects. Besides, the flexibility of shifting the production rate of SNG or ammonia makes it possible to adapt to changes in the market demand.