Production from shale gas reservoirs plays an important role in natural gas supply in the United States. Horizontal drilling and multi-stage hydraulic fracturing are the two key enabling technologies for the economic development of these shale gas reservoirs. It is believed that gas in shale reservoirs is mainly composed of free gas within fractures and pores and adsorbed gas in organic matter (kerogen). It is generally assumed in the literature that the monolayer Langmuir isotherm describes gas adsorption behavior in shale gas reservoirs. However, in this work, we have analyzed several experimental measurements of methane adsorption from the Marcellus Shale core samples that deviate from the Langmuir isotherm, but obey the BET (Brunauer, Emmett and Teller) isotherm. To the best of our knowledge, it is the first time to find that gas adsorption in a shale gas reservoir behaves like multilayer adsorption. Consequently, investigation of this specific gas desorption effect is important for accurate evaluation of well performance and completion effectiveness in shale gas reservoirs based on the BET isotherm. The difference in calculating original gas in place based on both isotherms is discussed. We also perform history matching with one production well from the Marcellus Shale and evaluate the contribution of gas desorption to the well's performance. History matching shows that gas adsorption obeying the BET isotherm contributes more to overall gas recovery than gas adsorption obeying Langmuir isotherm, especially early time in production. This work provides better understanding of gas desorption in shale gas reservoirs and updates our current analytical and numerical models for simulation of shale gas production.