They are not interchangeable, the meanings are as follows. Reject: Dismiss as inadequate, inappropriate, or not to one's taste. Refuse: To indicate or show that one is not willing to do something

They both can be used as verb and noun.First,let us talk about the usage as noun."refuse"=waste material that has been thrown away;"reject"=sth that can't be used or sold because there is sth wrong with it or a person who has not been accepted as a member of a team,society,etc.Then,about their usage of verb."reject‘ mainly refer to reject an argument/a claim/a desicion/an offer/an suggestoin."refuse"means you will not do sth someone has asked you to do.