We consider the optimal load-shedding problem in electric power systems where a number of transmission lines are to be taken out of service. The nonlinear power flow equations and the binary decision variables result in a mixed-integer nonlinear program. We show that the load-shedding problem has a separable structure when the power flow equation is relaxed. We exploit the separable structure by using the alternating direction method of multipliers. Numerical experiments on IEEE 118-bus system demonstrate that our approach significantly outperforms random selection of lines. Computational results suggest that removing transmission lines between load buses results in less load shedding in power systems.