The main objective of this work is to find the set of scenarios for variable feed conditions which the plant can accommodate operating in the feasible region. A rigorous simulation of the natural gas plant has been implemented. The solubility of carbon dioxide in mixtures of light hydrocarbons is predicted Carbon dioxide solidification in the coldest sectors of the plant is not desired. Therefore a constraint for each of the top plates of the demethanizer column is added, in such a way that the operating conditions are chosen to avoid freezing conditions. The nominal point that represents the normal feed conditions is known. The maximum displacements from the nominal point, in different directions of the uncertain space, are found solving a nonlinear programming problem. The main operating conditions of the plant are the optimization variables. In the numerical example presented, the carbon dioxide and methane feed flowrate are assumed as uncertain parameters. The minimum of the maximum displacement from the nominal point is found The corresponding direction and active constraints which limit the flexibility are reported The feasible region in the two dimensional space is found.