The monomer of polyethylene is ethylene - H2C=CH2. {eth=2 carbons, ene=a CC double bond} Two methane - H2CH2 or H3CH - molecules condense to form ethane: H3C-CH3 + 2 H+. Another 2 H+ removal, or condensation, gives the -ene H2C=CH2. Polymerization of the -enes is the next step. Via another condensation reaction, a poly-eth-ene appears thusly: H2C=CH|HC=CH|HC=CH|HC=CH|HC=CH|HC... & etcetera.