One of the most used methods for municipal solid waste (MSW) management is recovering energy from the organic fraction of municipal solid waste (OFMSW) through anaerobic digestion (AD) system. Not only in developed countries (with high source separation), but also in developing countries (with poor source separation), this energy recovery technology can be interested. In many countries with mixed MSW like Iran, a mechanical separation (trommel screen, magnet, etc.) is used as a simple and efficient method to separate OFMSW. In this study, three main goals are followed; firstly, Selection of a suitable AD system (wet anaerobic digestion (WAD) or dry anaerobic digestion (DAD) and mesophilic or thermophilic)) according to physical and chemical composition of under-screen fraction of MSW and climate conditions. Secondly, evaluation of biogas energy potential of under-screen fraction of Isfahan municipal solid waste (USFIMSW), as one of the major cities of Iran is followed. Finally, it offers to use the results obtained in other developing countries with similar conditions in Isfahan. The physical analysis shows that about 70% of Isfahan MSW consists of the organic fraction (OF) which can be used in the AD system. Also, the value of total solid (TS), volatile solid (VS) and thereby VS/TS for the under-screen fraction of MSW changes seasonally between about 29% and 35.5%, 21%–25.5% and 64%–84%, respectively. The results show that in cities with a low source separation (due to the presence of hazardous materials in the under-screen fraction of MSW), high TS and dry climate like Isfahan, the use of DAD is more appropriate. According to bio-methane potential (BMP), TS and VS analysis, the average amount of methane yield, without purification of USFSMSW, is about 322.7 Nm³/t VS which can be increased by various steps in pre-treatment, feedstock preparation and co-substrate.