Liquid-to-air heat exchangers are key components in residential cooling applications. They are typically made from metal (e.g., finned tube heat exchangers, such as water coils), but there has been a drive to use polymers due to the potential to make less costly products that are not as prone to fouling and corrosion as their traditional metal counterparts. The lower thermal conductivity of polymers, however, can compromise performance. Hence, design innovations must be considered to develop heat exchangers with comparable or improved performance since extended surfaces such as fins cannot be relied upon to enhance heat transfer. This research describes the design, fabrication, and experimental characterization of a few iterations of a novel, high-effectiveness Microchannel Polymer Heat eXchanger (MPHX). This heat exchanger implements a counterflow configuration that, while not prevalent in the literature due to the challenges associated with preventing water headers from obstructing airflow, has the potential to enhance the heat transfer over traditional water coils (which are arranged in crossflow). The microchannels for fluid flow increase the heat transfer surface area and thin walls address the issue of the lower thermal conductivity of polymers. Another advantage of the final iteration of the MPHX design presented is that it is easy to scale up or down to suit various applications. A sectional model accounting for potential condensation in the air stream is developed and validated with experimental data. The model predicts performance well for the first-generation MPHX, although it shows greater divergence from the second-generation MPHX, possibly due to limitations of the model and the experimental measurements. Overall, heat transfers of up to 433 W and 886 W, respectively, were achieved for the first- and second-generation MPHX units. A parametric study is performed to examine the impact of varying different parameters which could help inform future design iterations. The sectional model is implemented in a case study in conjunction with a Sub-Wet bulb Evaporative Chiller (SWEC) to assess the system performance of an energy-efficient cooling scheme for dry climate conditions. Additionally, a process-based cost model is developed for the MPHX to evaluate the viability and cost-competitiveness of the design, as well as identify opportunities to reduce the cost.