Mixed-convection flow and heat transfer of non-Newtonian power-law fluids over a pair of side-by-side circular cylinders in aiding buoyancy are investigated numerically. The calculations are performed in an unconfined domain for the following range of physical control parameters: Reynolds number (Re) = 1-40, Richardson number (Ri) = 0-1, and power-law index (n) = 0.2-1 for a transverse gap ratio of 1.5 at a constant Prandtl number (Pr) of SO. Extensive numerical results are presented, such as viscous, pressure, and total drag coefficients, average Nusselt number, and transition from a steady to a time-periodic regime. The overall drag coefficient and its components decrease with the Reynolds number and increase with the Richardson number for all values of the power-law index considered. The average Nusselt number increases with an increase in both the Reynolds and Richardson numbers but decreases with an increase in the power-law index. It is observed that aiding buoyancy and shear-thinning tendency augment heat-transfer characteristics. The maximum enhancement in heat transfer is found to be approximately 85% for Re = 40, n = 0.2, and Ri = 1. However, the maximum enhancement in the value of the average Nusselt number for Ri = 0 and 0.5 is found to be approximately 70% and 81%, respectively. Finally, a simple heat-transfer correlation is obtained for the values of Re, n, and Ri covered in this study.