A semi-analytical method is developed to investigate surface-wave interactions among an array of wave-energy converters, each modeled as a truncated cylinder, and the interaction effects on power absorption from the array is studied. Each cylinder can have independent movements with six degrees of freedom. The method of matched eigen-function expansions is applied to solve the wave radiation problem. To achieve fast computation, effects of evanescent modes of local scattering waves from one cylinder is neglected in the near fields of neighboring cylinders, but the far-field radiated waves are retained. Wave-exciting forces and moments on an individual cylinder or a group of cylinders, situated among an array, are evaluated by a new, generalized form of the “Haskind relation” applicable to an array of arbitrary configuration, which only requires the solution to the radiation problem. Hydrodynamic properties and wave-exciting loads are presented for arrays of different configurations. This efficient computation facilitates investigating wave-interaction effects on the optimal power output of a cylinder array. Effects of the cylinder numbers, their spacing, and the layout geometry on power extraction are discussed. The interaction factor for a large wave farm consisting of multiple small arrays was evaluated by the current method combined with the point-absorber approximation.