A fully 3-dimensional finite element model has been developed that simulates the internal temperature distribution of short-length high-power fiber lasers. We have validated the numerical model by building a short, cladding-pumped, Er-Yb-codoped fiber laser and measuring the core temperature during laser operation. A dual-end-pumped, actively cooled, fiber laser has generated >11 W CW output power at 1535 nm from only 11.9 cm of active fiber. Simulations indicate power-scaling possibilities with improved fiber and cooling designs.