Development of shale gas reservoirs has become an integral part of the North American gas supply. The Marcellus shale formation contains large amount of natural gas resources and its proximity to high demand markets along the East Coast of the United State makes it an attractive target for energy development. The economic viability of such unconventional gas development hinges on the effective stimulation of the low permeability reservoir rocks. Horizontal wells with multi-stage hydraulic fracturing treatments are the stimulation method of choice and have been successful in shale gas reservoirs. However, the fundamental science and engineering of the process are yet to be fully understood and hence the stimulation method needs to be optimized. In this paper, we used numerical reservoir simulation techniques and quantified the effect of the following pertinent factors on the well performances and long-term gas recovery: multiphase flow, proppant crushing, proppant diagenesis, reservoir compaction, capillary pressure, and operating conditions. The knowledge generated in this study enables engineers to better design fracture treatments and operators to better manage the wells in the Marcellus shale gas reservoirs.