At the World Championship 2014 his team beat France in 3rd place match and won bronze medal. In 2017 he took over Belgian national team. On 7 February 2018 Heynen was chosen as the new head coach of Poland men's national volleyball team. At the World Championship 2018 in Italy, Poland led by Heynen beat Brazil in the final, defended the World Championship 2014 title, and achieved 3rd World Champion title in total. In 2019 Heynen coached Poland to the European Championship 2019 bronze medal by beating France in 3rd place match and losing only to Slovenia in the semifinal. At the FIVB World Cup 2019 his team lost only against USA and Brazil and eventually was placed 2nd in the competition, winning the silver medal. In November, 2019 he became a head coach of Sir Safety Conad Perugia. In the 2019/2020 season Sir Safety Perugia coached by Heynen achieved the Italian SuperCup after defeating Modena Volley in the final.