HR is not issuing No Objection Certificate for some absurd reason

I am employed with a Maharatna CPSU at Noida since 2007. In view of better opportunities and serving options, I need to switch my job in other govt organisation. In case of joining in other govt orgn, it is required to produce 'No Objection Certificate' from the previous Govt employer. Now, my current employer is not issuing me NOC for the reason that they require substitute of equal capacity of mine. Procedure is like the NOC form is to be approved/ issued by HR Deptt. duly screened and commented by Deptt Head. There is a column in the Performa, which asks for requirement of substitute and the same has to be commented by Head. In case, if Head answers in yes, HR simply denies giving NOC to the employee in need. Such action bars me to exercise my fundamental right to freedom to practice any occupation under Article 19'g'. How can a company stop any employee seeking quit from the company just because of substitute?? It is simply absurd and non-nonsensical! Kindly advise in what way should I proceed to get NOC from current employer so that the same can be produced to another govt employer?