A 52" diameter flammable gas line!?!? This is nowhere for an unauthorized individual to work. Leave gas mains alone. If you think that your gas main has an obstruction that requires cleaning, contact the local utility.
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"Don't disturb my circles." translation of Archimedes last words
As other contributors have already pointed out, this is a high risk operation. Lives have been lost recently while cleaning fuel gas lines (google Kleen Energy). Partially in response to those disasters the NFPA has issued a new provisional standard, NFPA 56 Standard for Fire & Explosion Prevention during cleaning and purging of flammable gas piping systems. It may not be entirely applicable to your situation but contains some good "things to consider" when planning such an operation.
As to the best method of cleaning, that will be dependent on what materials you have available, system scope and layout, degree of cleaning required - and the potential hazards of each method. One method which is specifically excluded by the new standard is to use fuel gas itself as the cleaning medium - which is what was happening at Kleen Energy when it went wrong.