Optimized conditioning of Information and Communication Technology (ICT) centers containing sensible heat generating equipment is achieved by indirect air-side economizing. In this process, the conditioned primary air stream is recirculated through a plate-type cross-flow heat exchanger, in which the cross-flow consists of a completely segregated cooler secondary ambient air stream. The air-to-air cross-flow heat exchanger comprises a series of parallel square or rectangular plates, which define a series of orthogonally alternating air passageways. This cross-flow design effectively prevents the mixing or blending of the primary and secondary air streams and thus avoids the efficiency losses and process airstream cross-contamination due to leakage, which is inherent in wheel type heat exchangers. The unique modular tunnel design of the cross-flow plate heat exchanger arrangement offers unit scalability and adjustability for various capacities and space demands. Real-time sensing of thermal demands and variable capacity control, coupled with on-demand mechanical cooling and humidification provisions, facilitate continuous operational optimization in all demands and ambient conditions.