The volume of US exports of polyethylene that could potentially be exposed to punitive tariffs totaled 229,000 tons in 2017. Conventional high pressure low density polyethylene was  155,000 tons of that total. 

China identified polyethylene from the US by specific Harmonized System codes. By far the largest component was conventional high pressure polymer, accounting for two-thirds of the total. The HS code under which China imports linear low density polyethylene not included.

Some US exports of linear may fall under two of the codes China listed, but this would impact less than 74,000 tons of US exports, based on 2017 data.

From International Trader Publications: China Polymer Trade Report; and World Trade Reports on LDPE, LLDPE and Ethylene-Alpha-Olefins Copolymers.