This paper introduces a novel methodology for predicting the lifetime of nonmetallic materials exposed to downhole environments, including high and low pH, H2S, H2, CO2, and brines. The methodology was developed based on fundamental material principles, including viscoelasticity, hyperelasticity, and chemical/physical degradation from laboratory aging tests, field data, and finite-element analyses. This paper describes four case studies that show this methodology's effectiveness in defining the operational limits of polymeric materials. They cover 1) a fluoroelastomer O-ring from an oilfield tool; 2) Nitrile rubber (NBR) and Hydrogenated nitrile butadiene rubber (HNBR) sealing elements degraded by H2S from a well testing job; 3) an HNBR permanent element production packer; and 4) the CO2/H2 compatibility and rapid gas decompression (RGD) of typical elastomers for carbon capture and sequestration (CCS), and H2 well storage. The proposed methodology enables a more comprehensive prediction of the sealing performance of nonmetallic components under downhole operating  conditions regardless of shape or size. It can be extended to develop operational limits of nonmetallic parts in the energy industry.