The soviet model worked quite well, combining strong influence of propaganda and good technical development.
Keep the sciences of economy, philosophy, social sciences and so on under whatever rock you wish to keep them. Keep technical sciences shining, and keep the bright scientists to live in gilded cages - modern, bright, well-supplied towns where access is restricted. Let them keep their families, enjoy entertainment that is not politically-incorrect but genuinely enjoyable, without propaganda, reward for work well done - and keep isolated from the grim outside. And from time to time let them enjoy being the heroes of the nation in great parades, ceremonies, TV interviews.
Travel outside the cities would not be forbidden but frowned upon. A bright scientist would probably get a politbureau officer for escort, making sure the grass is painted green wherever the scientist visits; such trips would be discouraged by the superiors, probably limiting promotions or restricting access to government-subsidized luxuries.
Recruitment? Elite studies; school tests that pick brightest kids, government-sponsored scholarships, and specifically if the student succeeds and appears to be really bright and hard-working, he or she can bring the whole family to the "science town" - so the whole family would support such an attempt (they would also take care of all the "technical" jobs of the town; janitors, shopkeepers, gardeners, repairmen etc).
Of course the scientists aren't let loose to do as they will. Each institution has two directors, a technical one and a political one. One gives direction to research, production, manages the progress, the other assures the unit receives all the necessary equipment, resources, but above all, watches the crew and overrules any promotion decisions, raises and privileges, making any dissenters reconsider their stance.
Above all, keep the population isolated from the outside, and vice versa, to keep dissent levels low, and nevertheless, while acknowledging, promoting, rewarding the successes, never give the scientists any actual power. The politbureau would be the actual power, but leaving the leash loose enough so that the scientists never feel it.
And if one of the scientists really wants to make a difference? The way is open. Join the party. Climb the ladder of ranks, starting from the very bottom, right on par with workers and farmers. Prove loyalty, wisdom and charisma. If you are good enough - loyal enough - you can become the right hand of the Leader, and influence the country through wise advice. Still, there is no "shortcut/bridge" from "professor" to "Delegate of Party Association of [city]" or such. For political matters, Professor is equal to Plumber, and the political career path is completely orthogonal to scientific achievements!