It is generally not harmful lying supine after a full meal. This is provided one does not suffer from significant gastric reflux disease. By adopting a horizontal position after a full meal may worsen any existing acid reflux since it is much more easier for fluid or gastric acid to flow horizontally up the oesophagus.
Having said that, it does not do any good to the digestive system by lying down immediately after a full meal since this posture tends to slow down the gastric emptying resulting possibly in a slower digestion process. I would advise at least a 2hr interval before lying down after any meal.
