Finite-element analysis (FEA) is utilized to model the temperature distribution and heat-induced deformation of the first mirror of the coherent soft X-ray beamline at NSLS-II. The FEA results of the cooling design show a thermal bump on the area illuminated by the beam plus an overall mirror bending. The resulting slope errors are almost linear over the area where the power is absorbed and have a complex dependence on the mirror thickness. The linear change of the slope error means a constant convex radius of curvature whose defocusing effect can be corrected with a bendable mirror positioned downstream in the beamline. In this paper we discuss the design optimization of the mirror thickness using FEA as a tool.