On reacting ethene with HCl we get chloroethane. It is an addition reaction.
The reaction of alkenes with halogens is an example of the electrophilic addition reaction. Vicinal dihalides are the products formed when alkenes react with halogens such as bromine, chlorine. Hydrogen chloride and the other hydrogen halides add on in exactly the same way.
Hydrogen chloride adds to ethene to make chloroethane: