Flow past an isolated circular cylinder is numerically simulated, under the influence of aiding and opposing buoyancy. A modified velocity correction procedure is incorporated. Galerkin weighted residual formulation is employed for spatial discretization along with a second-order Runge-Kutta (R-K) time integration scheme. The influence of buoyancy on the Nusselt number, wake structures, temporal lift and drag forces have been studied. At low Reynolds numbers (for, e.g., Re=20-40), buoyancy opposing the flow could trigger vortex shedding. The degeneration of the Karman vortex street into twin vortices is numerically simulated for a heated cylinder. The two zones of vortex shedding and twin vortices are demarcated.