Heynen started out his career as a volleyball coach in his hometown club Noliko Maaseik. In 2005, he became assistant coach, and a year later he was head coach.
During his six-year tenure at the club, his team won four Belgian Championships, five Belgian Cups and four Supercups. Additionally, Heynen was twice named Belgian Coach of the Year, in 2009 and 2011.
Heynen was chosen as the new head coach of the Poland men's national volleyball team on February 7, 2018.
Poland is now one of the world's strongest teams. It has been the reigning world champion since 2014 and is ranked fourth in the International Volleyball Federation's (FIVB) world ranking.