A novel "ghost interface" expression for the surface tension of a planar liquid-vapor interface is derived in detail from consideration of the free energy of the system, and a methodology for utilization of this new technique is given. An augmented Monte Carlo computer simulation procedure is developed specifically for the ghost interface, including derivation of long-range corrections resulting from potential truncation and a modified Gibbs ensemble technique for the simulation of adjacent coexisting phases. Results generated from the ghost interface theory for the surface tension are presented and found to be in good quantitative agreement with those resulting from the Kirkwood-Buff equation. Applications of this new approach to curved and to supersaturated systems are also discussed. (C) 2004 American Institute of Physics.