In this thesis, the impact of the presence or absence of composition gradient on one of the wells in gas fields in southern Iran will be checked. The case is a multi-layered reservoir with a lean gas condensate. For this purpose, the compositional gradient model was built using actual data, as an input was entered reservoir simulator. By simulating a synthetic gas condensate reservoir using rock and fluid properties the data needed to draw well inflow performance curves were derived. IPR curves for different scenarios of the reservoir and in the presence of gradient composition or regardless of it were drawn and were compared. To apply compositional gradient effect the two solutions were adopted, after review, one of them showed the expected results. This study shows that not considered compositional gradient in near-critical reservoirs, such as Iran's southern gas fields, can lead to errors in prediction production process, errors in estimating initial hydrocarbon in-place and error in the process of reservoir simulation.